Insular carcinoma of the thyroid is situated morphologically and biologically in an intermediate position between the well-differentiated and undifferentiated tumors and presents a variable clinical course in a widely heterogeneous spectrum. The present cancer staging system (TNM) for thyroid cancer considers differentiated and undifferentiated tumors while ignoring this Intermediate type, which is also called poorly differentiated tumor. In addition to the limited data on this rare disease, some poorly differentiated thyroid tumors contain differentiated cancer areas at various rates. These factors may cause difficulties in estimating disease aggressiveness and prognosis. To solve this problem, various microscopic and immunohistochemical parameters can be assessed. In this paper we describe 3 patients affected by thyroid carcinoma with an insular component, who presented different clinical pictures. When these cases were examined, the TNM system failed in stage grouping for poorly differentiated thyroid tumors. Case 1 and case 2 had similar clinical stages according to the TNM staging system for differentiated tumors, but had different prognoses. Case 3, with more limited disease, had the highest rate of poorly differentiated areas but the lowest Ki-67 proliferation index. In conclusion, it is difficult to make claims about the clinical behavior and prognosis of thyroid carcinoma with an insular component based on the 3 cases reported in this study, but it can be speculated that there is a gap in the TNM system with regard to the staging of insular thyroid carcinoma. In this situation the assessment of microscopic and immunohistochemical features of the tumor may help to predict disease aggressiveness and patient risk. However, it is clear that there is a need for large-scale studies evaluating the prognostic importance of histopathological and immunohistochemical features in determining risk groups.